Elevated Photodiode with a Stacked Scheme

ABSTRACT

A device includes an image sensor chip having formed therein an elevated photodiode, and a device chip underlying and bonded to the image sensor chip. The device chip has a read out circuit electrically connected to the elevated photodiode.

This application is a divisional of U.S. patent application Ser. No.13/671,330, filed Nov. 7, 2012, and entitled “Elevated Photodiode with aStacked Scheme,” which claims the benefit of the following provisionallyfiled U.S. Patent application: Application Ser. No. 61/677,851, filedJul. 31, 2012, and entitled “Elevated Photodiode with Stacked Scheme,”which applications are hereby incorporated herein by reference.

BACKGROUND

Image sensors, and in particular Back Side Illumination (BSI) imagesensors are becoming increasingly popular and used in a variety ofapplications. As is the trend with integrated circuit technology, thetrend is toward smaller and smaller features for image sensors, to allowfor lower cost and greater packing density. When pixels pitch scale downto the sub micrometer range, the photodiode area is limited and thus itbecomes difficult to maintain the performance such as Signal to NoiseRatio (SNR), Quantum Efficiency (QE), sensitivity, and the like.

Elevated photodiodes may overcome some shortcomings in conventionalstructures and methods for making such structures. Specific processtuning is needed, however, for so-called elevated photodiodes, and thesame would be constrained by Application Specific Integrated Circuit(ASIC) process conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a first embodimentstructure in accordance with some exemplary embodiments;

FIG. 2 is a schematic cross-sectional view of a first embodimentstructure in accordance with some exemplary embodiments;

FIGS. 3 a through 3 c illustrate cross-sectional views of intermediatesteps in the manufacturing of the illustrative embodiment device in FIG.1 in accordance with some exemplary embodiments; and

FIGS. 4 a through 4 c illustrate cross-sectional views of intermediatesteps in the manufacturing of the illustrative embodiment device in FIG.2 in accordance with some exemplary embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare illustrative, and do not limit the scope of the disclosure.

A package including a first chip including elevated photodiodes stackedon a second chip and the method of forming the same are provided inaccordance with various exemplary embodiments. The intermediate stagesof forming the package are illustrated. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

Before addressing the illustrated embodiments specifically, aspects ofvarious illustrated and contemplated embodiments are discussedgenerally. Embodiments of the present disclosure provide for a stackeddevice including a sensor wafer with an elevated photoelectricalconversion layer (sometimes referred to herein as a photoelectronconversion layer) and a device wafer, to form an elevated photodiodeimage sensor. In some embodiments, image sensor wafer 100 and devicewafer 200 (FIG. 1) are stacked for face-to-face bonding. In otherembodiments, image sensor wafer 100 and device wafer 200 (FIG. 2) arestacked for face-to-back bonding. In some embodiments, isolationstructures are formed between pixels to improve cross-talk isolationperformance. Various embodiments of the present disclosure may providefor a high fill factor of the resulting image sensor. Immunity toelectrical crosstalk between adjacent pixels is an advantageous featureof some embodiments. Embodiments of the invention also provide for aflexible process tuning on each wafer.

Turning now to the illustrated embodiments, FIG. 1 schematicallyillustrates in cross-section view an illustrative embodiment device inwhich image sensor wafer 100 and device wafer 200 are stacked in aface-to-face bonding arrangement. Two pixel units 110 are illustrated onimage sensor wafer 100, although many more pixel units 110 are typicallyformed on image sensor wafer 100. One skilled in the art will recognizethe components of image sensor wafer 100, including the illustratedpixel units 110, and such components are not described in detail hereinfor clarity. Likewise, the components of device wafer 200, wherebyelectrical signals from pixel units 110 can be received and processedare schematically illustrated herein, but are not described in detail,as those details are not necessary for an understanding of the presentdisclosure and/or will be apparent to those skilled in the art onceinformed by the present disclosure.

FIG. 2 illustrates a stacked image sensor wafer 100 and device wafer200, wherein wafers 100 and 200 are configured in a face-to-back bondingconfiguration. Note the different configuration of image sensor wafer100, particularly the location of the storage nodes 112 and interconnectstructures 114, for the face-to-back bonding configuration.

Intermediate steps in the manufacture of a package, such as the packageillustrated in FIG. 1, are schematically illustrated in FIGS. 3 athrough 3 c. FIG. 3 a illustrates a structure in an intermediate stageof manufacture. At the stage illustrated in FIG. 3 a, image sensor wafer100, which includes a plurality of identical image sensor chips 100′therein, is formed. Storage nodes 112 and interconnect structures 114have been formed on sensor wafer 100, and image sensor wafer 100 hasbeen bonded in a face-to-face configuration to device wafer 200 (whereinappropriate active and passive devices and interconnects have beenpreviously formed). Storage nodes 112 are formed of implanted regions.In some embodiments, storage nodes 112 are connected to transfer gatetransistors 118, which are configured to electrically interconnect anddisconnect storage nodes 112 and corresponding floating diffusion region116 in the same pixel unit 110. In alternative embodiments, transfergate transistors 118 are formed in device wafer 200 rather than in imagesensor wafer 100. In the illustrated embodiments, interconnectstructures 114 are illustrated as including one level ofinterconnection. It is appreciated that a plurality of layers ofinterconnect structures 114 may be formed in a plurality of dielectriclayers 121, which may include low-k dielectric layers in someembodiments.

Image sensor wafer 100 also includes semiconductor substrate 120, whichmay be a silicon substrate, or may be formed of other semiconductormaterials such as silicon germanium, silicon carbon, III-V compoundsemiconductor materials, or the like. Throughout the description, theside of substrate 120 including interconnect structures 114 is referredto as the front side of substrate 120 (which side is also referred to asthe front side of image sensor wafer 100), and the opposite side isreferred to as the back side. Accordingly, in FIG. 3 a, the front sideof image sensor wafer 100 faces down.

Two pixel units 110 are illustrated on image sensor wafer 100 forsimplicity. Significantly more pixel units 110 are within thecontemplated scope of this disclosure. Pixel units 110 are separated byisolation features 124, such as Deep Trench Isolation (DTI) structures,which are formed of, or comprise, a dielectric material such as an oxide(silicon oxide, for example) and/or a nitride (silicon nitride, forexample). DTI structures 124 may extend from the back surface ofsubstrate 120 into substrate 120. Furthermore, in some embodiments, DTIstructures 124 may penetrate substrate 120, and extend from the frontsurface 120A (the surface facing down) to the back surface 120B ofsubstrate 120. In the top view of the structure in FIG. 3 a, DTIstructures 124 may be interconnected to form a continuous grid, withpixel units 110 include portions in the grid openings, and portionsoutside and vertically aligned to the grid openings of the grid.Alternatively stated, DTI structures 124 include a plurality of rings,each encircling a portion of each of pixel units 110.

FIG. 3 a also illustrates a cross-sectional view of device wafer 200,which comprises a plurality of identical device chips 200′ therein.Device wafer 200 includes substrate 220, and logic circuit 223 formed atthe front surface of substrate 220. Substrate 220 is a silicon substratein some embodiments. Alternatively, substrate 220 is formed of othersemiconductor materials such as silicon germanium, silicon carbon, III-Vcompound semiconductor materials, or the like. In accordance with someexemplary embodiments, logic circuit 223 includes read out circuits 222.Each of read out circuits 222 may include a plurality of transistors,such as row selector 226, source follower 228, and reset transistor 230.Row selectors 226, source followers 228, and reset transistors 230 mayform portions of pixel units 110, with each of pixel units 110 includingone of row selectors 226, one of source followers 228, and one of resettransistors 230. Accordingly, each of pixel units 110 may extend intodevice wafer 200 to include one of read out circuits 222.

Logic circuit 222 may also include one or more of Image SignalProcessing (ISP) circuits such as Analog-to-Digital Converters (ADCs),Correlated Double Sampling (CDS) circuits, row decoders, and the like,which may also be considered as parts of the read out circuits.Interconnect structure 214 is formed over, and electrically coupled to,logic circuit 223. Interconnect structure 214 includes a plurality ofmetal layers in a plurality of dielectric layers 221, with metal linesand vias disposed in dielectric layers 221. In some exemplaryembodiments, dielectric layers 221 include low-k dielectric layers. Thelow-k dielectric layers may have low k values lower than about 3.0.Dielectric layers 221 may further include a passivation layer formed ofnon-low-k dielectric materials having k values greater than 3.9. In someembodiments, the passivation layer includes a silicon oxide layer, asilicon nitride layer, an Un-doped Silicate Glass (USG) layer, and/orthe like.

Metal pads 142 and 242 are formed at the surfaces of wafers 100 and 200,respectively, wherein metal pads 142 and 242 may have their top surfacessubstantially level with the top surfaces of the top ones of dielectriclayers 121 and 221, respectively. Metal pads 142 and 242 may alsocomprise copper, aluminum, and possibly other metals. Metal pads 142 arebonded to the respective metal pads 242, so that the devices in wafers100 and 200 are electrically coupled to each other. In some embodiments,as a result of the bonding, each of the pixel units 110 includes aportion in wafer 100 and a portion in wafer 200, which are electricallyconnected to each other to form an integrated functional pixel unit thatmay generate electrical signals in response to photon stimulation, andstore and output the electrical signals in response to the commands forreading and resetting the electrical signals.

As shown in FIG. 3 a, the backside of image sensor wafer 100 is thinneddown (symbolized by arrow 145) after the bonding of wafers 100 and 200.The resulting semiconductor substrate 120 may have a thickness smallerthan about 10 μm, or smaller than about 5 μm. Image sensor wafer 100 canbe thinned down, e.g., by mechanical grinding/polishing, throughchemical mechanical polishing, through etching, or the like. Thethickness is an artifact of the technology node and the desired deviceproperties, and can be adjusted through routine experimentation.

As shown in FIG. 3 b, portions of the backside of substrate 120 arefurther thinned down to expose respective storage nodes 112 in therespective pixel units 110. The thinning down may be performed byselectively etching substrate 120, and DTI structures 124 are notetched. As a result, back surface 120B of substrate 120 is lower thanthe top ends 124A of DTI structures 124. Pixel electrodes 144 are formedin electrical communication with respective storage nodes 112. In someembodiments, pixel electrodes 144 are also referred to as bottomelectrodes 144.

While other conductive materials are within the contemplated scope ofthe present disclosure, examples of available materials for bottomelectrodes 144 include Al, TiN, Cr, and the like. Bottom electrodes 144can be deposited using techniques such as Chemical Vapor Deposition(CVD), Physical Vapor Deposition (PVD), Plasma Enhanced Chemical VaporDeposition (PECVD), Metal Organic Chemical Vapor Deposition (MOCVD),sputtering, and the like. In the illustrated embodiments, bottomelectrodes 144 are formed to a thickness of between about 0.3 μm andabout 0.8 μm. Bottom electrodes 144 may be in contact with therespective underlying storage nodes 112 to establish the electricalconnection.

Photoelectrical conversion layers 146 are formed over and in electricalcommunication with (and may be in contact with) bottom electrodes 144,as shown in FIG. 3 c. Transparent top electrode 148 is formed over andin electrical communication with photoelectrical conversion layers 146.By way of example, and not by way of limitation, photoelectricalconversion layers 146 could comprise amorphous silicon, a quantum dotlayer, an organic material, or the like. Likewise, top electrode 148could comprise Indium Tin Oxide (ITO), although one skilled in the artwill recognize suitable alternatives through routine experimentationonce informed by the present disclosure. Photoelectrical conversionlayers 146 may be separated from each other by DTI structures 124. Topelectrode 148 may be a continuous layer having its bottom surfacecontacting the top surfaces of photoelectrical conversion layers 146 andDTI structure 124. Each of Bottom electrodes 144, photoelectricalconversion layers 146 and the respective overlying portion of topelectrode 148 forms an elevated photodiode 149, which is different fromconventional photodiodes that are built in semiconductor substrates.

Referring back to FIG. 1, in accordance with some exemplary embodiments,additional components such as color filters 150, micro-lenses 152, andthe like, are further formed on the backside of image sensor wafer 100.The resulting stacked wafers 100 and 200 are then sawed apart intopackages, wherein each of the packages includes one chip 100′ from imagesensor wafer 100 and one chip 200′ from device wafer 200.

FIGS. 4 a through 4 c illustrate intermediate steps in the manufactureof a package, such as a face-to-back structure illustrated in FIG. 2.Unless specified otherwise, the materials and formation methods of thecomponents in these embodiments are essentially the same as the likecomponents, which are denoted by like reference numerals in theembodiments shown in FIGS. 1 and 3 a through 3 c. The details regardingthe formation process, the materials, and the specification, of thecomponents shown in FIGS. 2 and 4 a through 4 c may thus be found in thediscussion of the embodiment shown in FIGS. 1 and 3 a through 3 c.

As discussed above, image sensor wafer 100 that is used for aface-to-back configuration differs from image sensor wafer 100 suitablefor a face-to-face configuration. For example, as shown in FIG. 4 a,photodiodes 149 (including the transparent top electrode 148,photoelectrical conversion layers 146, and bottom electrodes 144) areformed over both substrate 120 and interconnect structure 114. Storagenodes 112 and respective photodiodes 149 are electrically connected toeach other through one or more intervening conductive interconnectstructure 114, as shown. Isolation structures 124′ in these embodimentsmay have top surfaces level with the top surfaces of photoelectricalconversion layers 146, and bottom surfaces level with the bottomsurfaces of bottom electrodes 144.

Image sensor wafer 100 is substantially complete in the stage ofmanufacture illustrated in FIG. 4 a with color filter 150 and/ormicro-lens 152 having been formed adjacent to photodiodes 149. Imagesensor wafer 100 is shown mounted to carrier 54, such as a glasssubstrate. This may be accomplished using adhesive layer 56 to adherecarrier 54 to image sensor wafer 100. Quartz, silicon, or othermaterials could be used in lieu of glass. After mounting image sensorwafer 100 to carrier 54, the backside of image sensor wafer 100 isthinned (symbolized by arrow 145), for example, using a process similarto those described regarding FIG. 3 a.

Note in these embodiments, it is not necessary to expose storage nodes112 in the wafer thinning process. This is partially because theelectrical contact to the storage nodes 112 is made from the top of thestorage node, i.e., the surface away from the backside surface of wafer100. After image sensor wafer 100 is thinned to a desired thickness,oxide layer 58 is formed on the backside, as shown in FIG. 4 b. In someembodiments, oxide layer 58 comprises silicon oxide, and may be formedusing a deposition method such as PECVD. Vias 60, sometimes referred toas through-vias 60, and sometimes referred to as through-substrate viasor through-silicon vias (TSVs) are also formed extending from thesurface of the backside of image sensor wafer 100 up to the interconnectstructure 114. Various electrical connections can be made in thismanner. Although one via 60 is shown in FIG. 4 b, multiple vias andhence multiple electrical connections can be formed. Furthermore, eachof pixel units 110 may include one or more via. Dashed lines 125 aredrawn to represent the likely electrical connections between vias 60 andthe devices in wafer 100.

FIG. 4 c illustrates the process of bonding the back side of imagesensor wafer 100 to the front side of a device wafer. Dashed lines 225are also drawn to represent the electrical connections between vias 60and logic circuits 223. Device wafer 200 may be essentially the same asillustrated in FIG. 3 a, for instance. In the embodiments of FIG. 4 c,however, it is desirable that device wafer 200 has an appropriatebonding surface on its front side (the side with interconnect structure214) to ensure adequate adhesion and bonding to image sensor wafer 100.This surface could be a treated oxide surface, a highly polished siliconsurface, or any other surface that provides for sufficient bonding withthe oxide layer on the “back” side of image sensor wafer 100. Careshould be taken to properly align image sensor wafer 100 and devicewafer 200 in the bonding process to ensure that contacts formed on therespective wafers 100 and 200 align with one another to form goodelectrical contact between the devices on the respective wafers 100 and200. Carrier 54 and adhesive layer 56 can then be removed, and asingulation is performed to saw the bonded wafers 100 and 200 into aplurality of packages, each including one chip 100′ from wafer 100, andone chip 200′ from wafer 200, resulting in the structure illustrated inFIG. 2 in accordance with exemplary embodiments.

The illustrated embodiments provide the advantageous feature that theprocesses of the image sensor wafers can be optimized for forming thepixel elements. Furthermore, processes can be optimized for forming thesupporting circuitry such as the read out circuits in the device wafers.This is advantageous since the image sensor wafers and the device wafershave different process requirements. Accordingly, by separating themanufacturing processes, the manufacturing processes of the image sensorwafers and the device wafers do not affect each other. Furthermore, byforming elevated photodiodes, the fill factors of the photodiodes may bemaximized since it is not affected by the metal routing or anytransistor in the respective wafer.

In accordance with some embodiments, a device includes an image sensorchip having formed therein an elevated photodiode; and a device chipunderlying and bonded to the image sensor chip, the device chip having aread out circuit electrically connected to the elevated photodiode.

In accordance with other embodiments, a device includes an image sensorchip and a device chip. The image sensor chip includes a firstsemiconductor substrate, an elevated photodiode overlying the firstsemiconductor substrate, and a first interconnect structure electricallycoupled to the elevated photodiode. The device chip is underlying andbonded to the image sensor chip. The device chip includes a secondsemiconductor substrate, and a read out circuit at a top surface of thesecond semiconductor substrate. The read out circuit is electricallycoupled to the elevated photodiode. The device chip further includes asecond interconnect structure overlying the first semiconductorsubstrate.

In accordance with yet other embodiments, a method includes forming anelevated photodiode over a semiconductor substrate of an image sensorwafer, and bonding together the image sensor wafer and a device wafer,wherein the device wafer includes a read out circuit electricallycoupled to the elevated photodiode.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming an elevated photodiode over a semiconductor substrate of an image sensor wafer; and bonding together the image sensor wafer and a device wafer, wherein the device wafer comprises a read out circuit electrically coupled to the elevated photodiode.
 2. The method of claim 1 further comprising: thinning a backside of the image sensor wafer down to expose a storage node in the semiconductor substrate; and performing the step of forming the elevated photodiode comprising: forming a bottom electrode over and contacting the storage node; forming a photoelectrical conversion layer over the bottom electrode; and forming a top electrode over the photoelectrical conversion layer, wherein after the step of bonding, the device wafer and the elevated photodiode are on opposite sides of the semiconductor substrate.
 3. The method of claim 1 further comprising: thinning a backside of the image sensor wafer; forming a through via from a back side of the image sensor wafer; and performing the step of bonding, wherein a back side of the image sensor wafer is bonded to the device wafer.
 4. The method of claim 1, wherein the step of forming the elevated photodiode comprises: recessing a back surface of the semiconductor substrate, until the back surface is lower than a top end of a deep trench isolation region to form a recess; forming a bottom electrode in the recess; and forming a photoelectrical conversion layer over the bottom electrode.
 5. The method of claim 4, wherein the step of forming the elevated photodiode further comprises forming a transparent top electrode over and contacting the deep trench isolation region and the photoelectrical conversion layer.
 6. The method of claim 1, wherein the elevated photodiode comprises a quantum dot layer.
 7. A method of manufacturing a device, the method comprising: providing a image sensor semiconductor substrate; and forming an elevated photodiode over the semiconductor substrate.
 8. The method of claim 7, further comprising bonding the semiconductor substrate to a device substrate in a face-to-back configuration.
 9. The method of claim 7, wherein the forming the elevated photodiode further comprises: forming a bottom electrode; forming a photoelectrical conversion layer over the bottom electrode; and forming a top electrode over the photoelectrical conversion layer.
 10. The method of claim 9, wherein the forming the elevated photodiode further comprises: recessing a back surface of the image sensor semiconductor substrate, until the back surface is lower than a top end of a deep trench isolation region to form a recess; and forming the bottom electrode in the recess.
 11. The method of claim 7, wherein the forming the elevated photodiode further comprises forming a quantum dot layer.
 12. The method of claim 7, further comprising: forming a color filter overlying and aligned to the elevated photodiode; and forming a micro-lens overlying and aligned to the color filter.
 13. The method of claim 7, wherein the forming the elevated photodiode further comprises forming a quantum dot layer.
 14. A method of manufacturing a device, the method comprising: providing a semiconductor substrate with a storage node; and forming a photodiode over and separated from the semiconductor substrate and in electrical contact with the storage node, wherein the forming the photodiode further comprises: forming a bottom electrode in electrical connection with the storage node; and forming a photoelectrical conversion layer over the bottom electrode.
 15. The method of claim 14, further comprising bonding the semiconductor substrate to a device substrate, the device substrate electrically connected to the photodiode.
 16. The method of claim 15, wherein the bonding the semiconductor substrate to the device substrate bonds the semiconductor substrate and the device substrate in a face-to-face configuration.
 17. The method of claim 15, wherein the bonding the semiconductor substrate to the device substrate bonds the semiconductor substrate and the device substrate in a face-to-back configuration.
 18. The method of claim 14, wherein the forming the photodiode further comprises forming a recess surrounded by deep trench isolation structures and forming the photoelectrical conversion layer within the recess.
 19. The method of claim 14, further comprising forming an electrode over the photoelectrical conversion layer.
 20. The method of claim 14, wherein the forming the photoelectrical conversion layer further comprises forming a quantum dot layer. 